We have obtained evidence for regulatory subsets within a population of helper T cells which are carrier specific, H-2 restricted and require associative recognition of hapten-carrier conjugates for effective cooperation with specific B precursors. Subsets of these T cells can be detected because they interact with matching B cells which differ in the immunoglobulin variable regions they express. There is good reason to believe that the particular subpopulation of helper T cells we have analyzed does not include the antigen activated, idiotype specific T cells which have been detected by others. We present a model to account for cooperation subsets of H-2 restzicted, carrier specific T cells which evolves from consideration of the basis for the antigen specificity of histocompatibility-linked Ir genes. In brief, we propose that each individual expresses multiple Ir genes. A set of molecules coded for by these Ir genes in B cells and a set of receptors specific for these molecules in T cells are clonally distributed to form cooperation subsets. The basis for the specificity of Iz genes is that unique germ line genes for the variable region of antigen receptors are expressed in specific T cells only in association with the receptor for a unique Ir gene product. Methods are described for clonal analysis of the progeny of isolated precursors of helper T cells. We propose experiments using these methods to determine the genetic basis for cooperation subsets of carrier specific T cells and to investigate the relationship between these T cells and other subpopulations of helper T cells.